In WO2010/072198 A1 rain detection is described with the aid of a camera, which is used for automotive driver assistance functions. For rain detection bifocal optics are used, which form a sharp image of a portion of the windscreen onto a portion of the image chip or image sensor of the camera.
A disadvantage of this idea is the fact that an additional optical element is introduced, whose edge causes serious disturbances both in the beam path for the rain sensor area of the image chip as well as in the region for the driver assistance functions in the vicinity of the edge. In particular for implementations with small dimensions, the focus conditions for the driver assistance and the rain sensor area are strongly different, what must be compensated by an increased thickness of the optical element, thus leading to increased disturbances and a broad, non-usable region on the image chip around the edge.
Another disadvantage results from different pane inclinations, which have different optical distances between rain sensor detection area on the image chip and corresponding rain sensor surface on the pane. To continue to ensure a sharp optical imaging, for each modified installation situation the thickness of the optical element must be adapted.
In order to detect raindrops also at night, it is proposed in WO 2010/072198 A1 to couple light via a coupling element into the windscreen and to guide it via total reflection in the pane. By a decoupling element the totally reflected light is decoupled in the direction of the camera. When there are water drops on the windscreen, a part of the light is decoupled and is no longer totally reflected to the decoupling element. It is also disadvantageous here that for each modified pane inclination the integrated camera lighting unit must be mechanically adapted to the modified installation condition.
In U.S. Pat. No. 7,259,367 B2 also by means of a camera rain sensing is proposed, which provides a large-area lighting of the passing-through window of the camera opening angle with the pane. The camera focus is set to almost infinite and thus can be simultaneously used for driver assistance applications. Because of the imaging on the far range raindrops are noticeable only as disturbances in the image, which are detected by complex differential measurements of the images recorded with light pulsed or modulated in synchronization with the pixel clock.
However, computer simulations and measurements show that with this type of lighting only a very small portion of the light is reflected at the raindrops back into the camera. This fact leads to a poor signal to noise ratio and consequently to an insecure rain detection.